I describe and defend the view in a philosophy of mind that I call 'Normative Essentialism', according to which propositional attitudes have normative essences. Those normative essences are 'horizontal' rational requirements, by which I mean the requirement to have certain propositional attitudes given other propositional attitudes. Different propositional attitudes impose different horizontal rational requirements. I distinguish a stronger and a weaker version of this doctrine and argue for the weaker version. I explore the consequences for knowledge of mind, and I then consider objections to the view from mental causation, from empirical psychology, and from animals and small children.